There is significant interest in developing tunable, pulsed lasers. It is well known that the pulses in these lasers can be compressed or chirped by controlling the dispersion effects in the laser cavity. Controlling the amount of negative group velocity dispersion (GVD) in a cavity can be particularly important if soliton-like pulse shaping is desired.
Various optical elements have been used to control the dispersion effects in a laser cavity. For example, a pair of separated prisms have a negative group velocity dispersion which can be varied by varying the position of the prisms with respect to the traversing laser beam. (See U.S. Pat. No. 4,727,553 issued Feb. 23, 1988 to Fork) For high levels of negative group velocity dispersion over a narrow bandwidth, a Gires-Tournois interferometer can be used. A Gires-Tournois interferometer is, in essence, a type of etalon. Details concerning the use of a Gires-Tournois interferometer both within and outside of a resonant cavity to control pulse formation can be found in "Intracavity chirp compensation in a colliding pulse mode-locked laser using thin-film interferometers," Heppner and Kuhl, Applied Physics Letters, Vol. 47, No. 5, Page 453, Sept. 1, 1985 and "Tunable group velocity dispersion interferometer for intracavity and extracavity applications," French et al, Optics Communications, Vol. 57, No. 4, page 263, Mar. 15, 1986.
As described in the latter articles and illustrated in FIG. 1 herein, a basic Gires-Tournois interferometer 2 consists of a substrate 3 having an index of refraction greater than air. The surface 4, upon which the laser beam 5 is incident, may be coated to be partially reflecting of the light. The rear surface 6 of the substrate is coated with a high reflecting coating. The magnitude and sign of the group velocity dispersion of this structure is varied by tilting the substrate with respect to the incident beam so that the path length of the beam within the substrate is changed. The structure can be used as an end mirror or an intermediate mirror in a laser cavity.
Another type of Gires-Tournois interferometer 10 is illustrated in FIG. 2 and consists of pair of separated plates 12 and 14. The surface 15 of plate 12 upon which the laser beam is incident must be highly transmissive. High transmission is achieved either with an antireflection coating or by orienting the surface at Brewster's angle. The inner surface 16 of plate 12 is provided with a relatively low level of reflectivity. The inner surface 18 of plate 14 is defined as a high reflector. In this version of the Gires-Tournois interferometer, the GVD is varied by changing the spacing between the plates.
In both of the interferometer structures shown in FIGS. 1 and 2, the group velocity dispersion is also dependent upon the wavelength of light incident thereon. FIG. 3a is a graph plotting the variation of the group velocity dispersion of a Gires-Tournois interferometer with respect to the wavelength of the incident light. As can be seen from curve 30, this dependence is periodic in nature. The variation in group velocity dispersion with wavelength poses a significant practical problem when using a Gires-Tournois interferometer in a tunable pulsed laser. More specifically, in order to form highly stable soliton-like pulses, the group velocity dispersion must be accurately controlled and maintained. Unfortunately, as the wavelength of the laser is tuned, the GVD of the Gires-Tournois interferometer will vary. This effect will cause the pulse formation to become unstable to the point that the laser will no longer operate in the pulsed mode. In order to reestablish pulsed behavior, the Gires-Tournois interferometer would have to be readjusted to achieve the desired GVD for the selected wavelength. Accordingly, it would be desirable to create a system for maintaining the desired GVD of a Gires-Tournois interferometer as the wavelength of the laser is tuned.
Another issue of concern in broadly tunable, short pulsed lasers is the selection of resonator optics. More particularly, consideration must be given to selecting coatings which will not create their own dispersion effects as the laser is tuned. Under present day coating technology, coatings which are relatively dispersion free are also relatively narrow band with respect to the available tuning ranges. For example, a laser having a Ti:sapphire gain medium is tunable from about 670 nm to 1.08 microns. In the present commercial embodiment of Coherent's Ti:sapphire lasers, three complete mirror sets are provided to cover the tuning range. For example one set will cover 670 to 800 nm, the next set 800 to 940 nm and the third set is used to cover the remainder of the range.
If a customer wishes to tune the laser over the entire range, the entire mirror sets must be changed twice. In at least one model of Coherent's laser, there are a total of nine mirrors in the resonator. Even the most experienced technician requires at least 30 minutes to change and realign a nine mirror set. For most customers who perform the task less frequently, changing the mirror set can take hours. Clearly, it would be desirable to be able to utilize mirrors with broader bands of reflectivity.
A typical narrow band reflector is formed from a series of alternating layers of high and low index materials each having an optical thickness of a quarter wave associated with the center frequency of the narrow band. For coatings having a reflectivity range significantly broader than 100 nm, at least two quarter wave stacks are used. The first stack is optimized for one wavelength while the second stack is optimized for another wavelength. Alternatively, a broad band coatings can include layers where the optical thickness progressively changes. Such broad band coatings have been used in steering optics outside the laser. However, the prior art literature clearly teaches that such multistack coatings should not be used inside the resonator of a short pulsed laser because the dispersion effects they create will broaden the pulses and inhibit stable operation. Accordingly, it would be desirable to design a mode locked laser system which permitted the use of resonator optics with broad band coatings so the mirrors sets would not have to be changed over the tuning range of the laser.
Therefore, it is one object of the subject invention to provide a method of automatically adjusting a Gires-Tournois interferometer.
It is a further object of the subject invention to provide a method of automatically maintaining the group velocity dispersion of a Gires-Tournois interferometer.
It is another object of the subject invention to provide a method of operating a tunable pulsed laser wherein the wavelength of the laser can be tuned without disruption of the mode locking process and without varying the duration of the pulses.
It is still another object of the subject invention to provide a mode locked laser having broad band reflective optics.
It is still a further object of the subject invention to provide a mechanism for compensating for the dispersion effects of a broad band optical coating used in the resonator of a pulsed laser.
It is still another object of the subject invention to utilize a Gires-Tournois interferometer to compensate for the dispersion effects created by a broad band reflective optical coating.